The present invention relates to a non-synchronous dc-dc converter having a bootstrap capacitor and a control circuit and, more particularly, to a method for controlling the charge of a bootstrap capacitor for non-synchronous type DC-DC converter, and also to an apparatus for controlling the charge of a bootstrap capacitor for non-synchronous type DC-DC converter.
In an integrated step-down dc-dc converter system, the power device is usually NPN transistor in bipolar technology, NMOS in CMOS technology or N-type DMOS in BCD technology. Bootstrapping is often used to raise the driver voltage above the power rail in order to reduce the loss of the top power transistor. P-type transistor is not commonly used due to the relatively large area needed, which is about 2-3 times larger than the n-type counterpart.
For a non-synchronous DC-DC converter, in normal operation when load current is high enough, the bootstrap capacitor is charged by a voltage regulator through a diode when the switching node of the dc-dc converter goes to (−Vd) at the off-time of the power transistor, where Vd is the forward diode voltage drop. When the power transistor turns on, the switching node will go high. The stored voltage in the bootstrap capacitor will generate a voltage higher than the supply voltage. However, when the load current becomes light, which means that the inductor current becomes discontinuous during the off-time of the power transistor, the switching node of the dc-dc converter will only stay at (−Vd) for a very short time, and will be at the same voltage level as the output when the inductor current becomes 0. As a result, the voltage stored in the bootstrap capacitor will eventually become the difference between the voltage regulator output voltage subtracted by forward diode voltage drop and the output voltage. This may not be enough to fully turn on the driver for the power transistor.
To overcome this problem, one method is to connect a dummy load at the output of the dc-dc converter, so that there will be a certain amount of current flowing in the dc-dc converter even at light load or no load condition, which would keep the output transistor switching. The dummy load should be of certain value to make sure that the switching node could be able to stay at (−Vd) for sufficient long time for the bootstrap capacitor to charge up. One problem with this simple method is that the efficiency of the dc-dc converter is suffered because the current in the dummy load always flows to ground, even when the bootstrap capacitor voltage is high enough.
FIG. 1 is a block diagram showing a conventional DC-DC converter with the PWM controlled output stage using bootstrap and dummy load. Here the bipolar power transistor 3 is used for illustration only. The transistor could be a NMOS or N-type DMOS transistor. The bootstrap capacitor 7 is connected between the cathode of the diode, 9 and the switching node, LX. A voltage regulator, 17, is connected to the anode of the diode 9. The voltage regulator charges the bootstrap capacitor C1 when LX goes to (−VD1) at the power transistor off-time. The voltage across the bootstrap capacitor Cboot is (VREF). When the power transistor 3 is turned on, LX will go high. Hence the voltage at BS becomes (LX+VREF), which can be higher than the power rail voltage and provide this voltage to the driver block and fully saturate the power transistor 3.
One problem of the conventional bootstrap method is that the bootstrap capacitor may not be fully charged at light load or no load conditions. When the load current is light, the inductor current will eventually becomes discontinuous. As can be seen from FIG. 2, at light load, LX stays at −Vd for T1 and then becomes Vout when the inductor current becomes 0. That means the time to charge up the bootstrap capacitor becomes T1. For the rest of the off-time, the voltage across the bootstrap capacitor is (VREF−VD2−Vout). When the Vout is close to VREF, or even worse when Vout is higher than VREF, there may not be enough charging to the bootstrap capacitor.
The situation becomes worse when there is no load current drawing from the output of the DC-DC converter. LX will stay at the same voltage as Vout for an even longer time. So the bootstrap capacitor could not be charged to an enough high level to turn on the driver circuit when the load current becomes heavy after the light load condition and the switching of the power transistor is required.
To overcome this, one method is to add a dummy load, as shown in FIG. 1. The purpose of the dummy load is to maintain a certain amount of current flow in the inductor to keep the switching of the output node. But this method will reduce the efficiency of the DC-DC converter for all the conditions.